Not all instability lines that reach the mature or squall-line stage develop in advance of a fast-moving cold front. The Great Plains region of the United States has a high frequency of these squall lines. The Great Plains type of squall lines also develop in warm, moist, unstable air masses. The necessary lifting or trigger may be supplied by intense thermal heating, orographic lifting, or convergent winds associated with a low-pressure area. 

FORMATION. The Great Plains squall line forms when an extremely unstable condition developsnormally in spring and summer. Ex-tremely unstable conditions exist when moist mP air cools in the upper levels because of the evaporation of falling precipitation. This cooler air aloft then moves over warm moist mT air (or even warm, moist, highly modified mP air) at the sur-face. If a sufficient trigger such as a steep lapse rate of a lifting mechanism exists, this extremely un-stable situation rapidly develops into a squall line. 

WEATHER. The weather associated with the Great Plains squall line is the same as that found with the prefrontal squall line. Because of the extreme instability, tornadoes are a common occurrence.

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